ok, I've made several potato rifles. All of which use PVC pipe, an explosive gas, and an ignitor. You simply place an object in the 1/2" barrel, then fill the combustion chamber with Oust air freshener or Axe body spray. Close the lid and press the ignitor and the object is launched at high speeds out of the barrel. Pretty neat.

But I heard that you can do the similar thing, but with 100 psi in the chamber, a ball valve and a valve stem. I built one with a valve stem completely sealed within the chamber.

but...

as I was filling it, the end cap BURST right off the cannon!!! It flew straight at my hand. My entire hand was completely numb and bleeding in several spots. I found the cap about 100 feet from the cannon.

OUCH! my hand still hurts, no broken bones though.

YAY, I just finished it and it works great. I shot a bouncy ball straight up and it was in the air for about 12 seconds. I shot a C battery out of it at a cynder block and it crushed the battery to half thesize.

very powerfull. I am operating it at 80 psi and still no exlposion of the cap.

How are you measuring your chamber pressure?
I've been reloading ammunition for more than 35 years, so I have more than a passing knowledge about pressures, as related to launching projectiles.
You should know that, often, it doesn't take much variance to create resistance, and resistance can boost chamber pressures enormously.
Be careful with that thing.
Sounds like fun but you may wish to create a remote ignition for it, so you're a safe distance away when it's fired. If it can hurt your hand, it can put out an eye or even kill.

sweet.....make a video of it and post it on you tube then give us the link
thanks